Referring to FIG. 1, a well contains a casing 10. The casing 10 lines a wellbore (not shown) and is cemented in place. A pump 12 is located inside the casing 10, frequently at great depths below the surface of the earth. The pump is used to pump production fluid from the depths of the well up to the surface. A shaft (not shown) connects pump 12 to motor 16. Production fluid enters the pump inlet 17 and is pumped out through tubing 18.
The motor tends to produce heat that must be removed to prolong the life of the motor. External devices used to decrease heat create additional costs. External cooling devices, for example, use a coolant pump above the well and coolant lines running through the wellbore to the pump. These cooling devices cool the pump by circulating the coolant through the pump and transferring the coolant back to the surface. The coolant pump, coolant lines, and coolant all create additional costs. Furthermore, the coolant lines may interfere with well operations.
The motor-pump assembly is located inside a wellbore so it is desirable to transfer heat to the production fluid that is flowing past the motor. It is common to arrange the pump and motor such that the production fluid flows past the motor on its way to the pump. Heat is transferred to the production fluid and carried away as the production fluid moves to the surface. It is desirable to increase the rate of heat transfer from the motor to the production fluid.
One method to increase the rate of heat transfer is to increase the surface area of the pump that is in contact with the production fluid. This can be done by elongating the motor housing or attaching a shroud to the pump or motor. The production fluid flows between the motor and the shroud so that heat can move from both the motor and the shroud into the production fluid. Other devices, such as fins, may be used to increase surface area of the motor. All of these methods of increasing surface area are limited by the small space available inside the wellbore. Furthermore, there is a problem with fins breaking off and creating blockages within the production fluid flow.
Fins may be used to create vortices within the production fluid. The vortices in the production fluid increase the rate of heat transfer between the motor and the production fluid. Unfortunately, the vortice-inducing fins, like fins used to increase the surface area, can break off and obstruct fluid flow. Fins also make pump manufacture and maintenance more difficult because they interfere with the assembly, disassembly, and the movement within the wellbore of the pump assembly.
Assembly is more difficult because the fins must be installed on the motor before the motor is inserted into the cylindrical shroud. The difficulty arises because the fins tend to interfere with the fit between the motor and the shroud. The height of the fins must be limited to allow for insertion, but even with a limited height they can still catch on other fins, the sides of the motor, or the wellbore. If the fin is attached to the motor, for example, there must be a gap between the outer edge of the fin and the shroud to allow clearance during assembly. Clearance issues also make it extremely difficult to attach fins to both the motor and the shroud in the same assembly because the fins interfere with each other during assembly and disassembly. Furthermore, fin clearance issues prevent the fin from spanning the entire gap between the shroud and the motor.
It is also difficult to perform maintenance on the motor when fins are attached directly to the motor housing because the fins make it more difficult to put the motor on a flat surface or hold it in a vice. In addition to increased assembly and maintenance costs, there is a cost associated with manufacturing and attaching the fins to the shroud and pump. It is desirable to increase the rate of heat transfer without incurring the disadvantages of fins.